1. Field of the Invention
This invention relates to a radar and in particular to a radar, which can suppress influence of noise caused by a power source for supplying drive power and check if the radar operates normally.
2. Description of the Related Art
In recent years, to improve safety during an automobile running, the number of automobiles each equipped with an FM-CW radar, which can detect distance to a vehicle running ahead of the automobile and relative speed between the vehicle running ahead and the automobile, has grown. The FM-CM radar transmits a frequency-modulated continuous wave forward, receives wave reflected by a vehicle to be detected, and detects the distance to the vehicle running ahead of the automobile and the relative speed between the vehicle running ahead and the automobile based on beat wave obtained by mixing the transmission wave and the reception wave.
Since the vehicle-installed radar as described above uses a battery as a power supply, a so-called DC-DC converter is generally provided for the power supply to provide power of various voltages required for the radar. The DC-DC converter once converts DC power of a predetermined voltage supplied from the battery into AC power and changes the voltage of the AC power by a transformer and then again converts the AC power into DC power of a necessary voltage. In order to control a switching element of an inverter, an oscillator for generating switching signals is required. In many cases, the fundamental wave of the switching signals or the harmonic of the switching signals exists in a 1-MHz band.
However, signals of an intermediate frequency circuit of a reception section of the radar are usually in the 1-MHz band. Therefore, the fundamental wave of the switching signals or the nth-order harmohic of the switching signals of the DC-DC converter may be superimposed on the intermediate-frequency signals as noises. FIG. 1 is a block diagram of a radar according to a related art. The radar includes a radar section 1, a DC-DC converter 15, and a switching signal oscillator 16.
The radar section 1 includes a transmission section 10 for generating a transmission wave, a heterodyne reception section 11 for extracting a baseband signal from a reception wave, a data processing section 12 for extracting information from the baseband signal, and an antenna 13 for radiating the transmission wave output from the transmission section 10 and receiving wave reflected by an object to be detected. The reception section 11 includes a high-frequency amplification section 110 for amplifying the reflected wave received at the antenna 13, a local oscillator 112 for converting into an intermediate frequency, an intermediate-frequency signal generation section 113 for mixing output of the high-frequency amplification section 110 and output of the local oscillator 112 to generate an intermediate-frequency signal, an intermediate-frequency signal amplification section 114 for amplifying the intermediate-frequency signal, and a detection section 115 for extracting the baseband signal.
FIGS. 2A and 2B are signal spectral drawings (No. 1). FIG. 2A shows the spectrum of the input signal of intermediate frequency generation section and FIG. 2B shows the spectrum of the output signal of the intermediate frequency generation section. That is, in the spectrum of the input signal of the intermediate frequency generation section, spectrum of fDD×n (n=1, 2 . . . ), which is the fundamental wave or harmonic of the switching signal of the DC-DC converter (noise), exists in a band of frequency fSW±ft, which is determined based on the upper limit of the detection range of the radar and has oscillation frequency fSW of the local oscillator 112 as its center.
Here, the frequency ft is a half of a sampling frequency, which is used when the baseband signal output from the detection section 115 is sampled and input into the data processing section 12. The frequency ft is determined based on the upper limit of the detection range of the radar. In the spectrum of the output signal of the intermediate frequency generation section, noise spectrum (fDD×n−fSW) exists between frequency band DC and ft.
In this case, it is inevitable that the radar will recognize the frequency of the noise component as a detection object although not actually exist and will provide a user with erroneous information.
Then, to solve the above-described problem, JP 2001-91631 discloses a radar, which does not output erroneous information caused by a DC-DC converter and checks if the radar operates normally.
That is, in the radar according to JP 2001-91631, the DC-DC converter 15 is excited by an output signal (local oscillation frequency) of the local oscillator 112. Thereby, frequency of the nth-order harmonic of the switching frequency fDD of the DC-DC converter 15, that is, fDD×n (n=1, 2 . . . ), is set to be identical with the local oscillation frequency fSW of the local oscillator 112, to thereby suppress occurrence of noise caused by the nth-order harmonic of the switching frequency of the DC-DC converter 15.